The objective of this high risk/high impact research proposal is to exploit the Cdo "knockout" mouse developed in this lab as a model for mild forms of holoprosencephaly (HPE), a human birth defect that affects craniofacial and forebrain development. Haploinsufficiency for Sonic hedgehog (Shh) is the most common known cause of both familial and sporadic HPE. The phenotype of Shh mutation carriers can be highly variable, even within a single pedigree, with craniofacial malformations ranging from cyclopia and a proboscis to absence of the nasal septum and a solitary median maxillary incisor. Although much is known about how Shh signals are transduced by target cells, few of the genes that this pathway regulates during craniofacial development have been identified. Cdo and Boc encode co-components of a cell surface receptor that promotes differentiation of skeletal muscle precursor cells. Both genes are also expressed in developing facial structures affected in HPE. We have disrupted the Cdo gene in mouse ES cells and introduced this mutation into the germline. Mice lacking CDO display highly penetrant defects in craniofacial development that are strikingly similar to those observed in milder forms of HPE in humans, including lack of, or solitary central, maxillary incisors, and lack or hypoplasia of the cartilage of the nasal septum. A hypothesis based on a causal relationship between Shh signaling and Cdo and/or Boc expression is proposed for the development of facial anomalies in mild form HPE: Shh produced by the ectoderm of the developing frontonasal and maxillary processes induces the expression of Cdo and/or Boc in the adjacent mesenchyme. Subsequent activities of the CDO/BOC receptor are required for the determination, differentiation or survival of cells in this region, ultimately resulting in formation of specific midface structures. It is predicted that disruption of Shh-mediated expression of Cdo and/or Boc results in facial anomalies characteristic of mild HPE. The specific aim of this proposal is to test this hypothesis with a combined embryological, genetic and cell biological approach. If the hypothesis is proven correct, the impact of this grant will be of great innovative value for the cell-signaling field and may lead to identification of molecular targets ultimately used for the diagnosis, prevention or treatment of mild HPE. [unreadable] [unreadable]